USB Insight Hub

A USB interfacing tool for developers & tech enthusiasts

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Nov 06, 2024

Project update 2 of 5

Understanding the Power Path

by David S

A big thank you to all the backers who have shown support for the project so far!

This first update is quite technical, but it covers one of the most important features of USB Insight HUB, a feature that sets it apart from other commercial solutions: the power path circuitry. As part of an open source hardware project, it’s important I disclose the rationale of “why” and “how” a certain solution was implemented and, in this way, improve, find weakness, or find inspiration for better alternatives. Feel free to ask any technical question through the Ask a Question link on our campaign page.

Why the Power Path is Important

Over many years working with embedded systems, I’ve learned to always account for the presence of USB devices: tools, products in production, development cards, etc. This versatility is incredible but not without risks: short circuits, surges, or overloads can damage a computer (I’ve seen some ports die along the way), especially when making connections with equipment with unknown faults or that are undergoing active development.

For these reasons, during the inception of the USB Insight Hub, one of the most important goals was to provide a robust power path within the system. But what exactly does “robust” mean anyway? I gave myself the following requirements based on a set of specific reasons:

RequirementRationale
Overvoltage protection for the power inputs and outputsProtection for the internal hub components and downstream devices. Surprisingly, several off-the-shelf hubs I inspected lacked this protection. Basically they trust the power provided through the port to be in a safe range and just pass the input voltage through to the downstream devices.
Reverse input voltage protection.A reverse voltage in a circuit simply fries all the electronics. A common and effective solution is to place a diode in series. This protection is mostly found in an external power supply input (if present).
No output should be able to-feed power back to any input or any other output.This is important, but often overlooked in simple hubs: imagine you connect a USB device that is self-powered, say by a battery or an external supply, to a downstream port of the USB hub. If the voltage of this device is higher than the voltage of the hub, the current will start to flow back from the device to the hub until the whole USB hub power rail is powered by the downstream device, thus back-feeding all the other devices connected to downstream ports and even the host (upstream). This can cause anything from minor inconveniences (the hub stays powered on when upstream power is removed) to the destruction of the host computer due to reverse current.
Provide the option to power the hub using host power (i.e., a computer) or from an external source via a USB-C connector.This is a common and necessary feature desirable in a USB hub. USB downstream ports have a limit to the current they can supply: 500mA for USB 2.0 and 900mA for USB 3.0 (by spec, though in reality many hosts can supply more current). If you need to source more current, an external power supply is required. In most cases the connector used is a barrel jack (in various sizes) or more exotic methods like screw terminals.
Handle up to 5 A total current. 2 A for each downstream port.This limit is a self-imposed value to match with higher wattage commercial power supplies with a USB-C connector that can supply up to 5 V (e.g., Raspberry Pi 27W USB-C).
Measure current and voltage of the downstream device.Initially, this feature was not contemplated. But I noticed how frequently I use a USB power meter (great tool by the way) so incorporating one to the Insight Hub added a lot of value. Later, it proved invaluable for improving the over-current and reverse current functionality

The simplest solution and Its Problems…

Looking at a simplified diagram, the solution seems simple, right?

Graph One: Simple solution for the power path.

There are several drawbacks with this simple solution, though:

  1. The biggest one is that, from input to output, there are two diodes in series. Even using the best Schottky diodes means that, at 2 A current, there is a 0.5 – 0.6 V drop. As we are working with a 5 V system, this means a 10% voltage drop (with the subsequent power dissipation). And, this is only accounting for the diodes' dropout, but it must be considered that there are also losses from the electronic switches, fuses, current sensing resistor, PCB and cables.
  2. The USB standard limits the in-rush current in the first connection to an equivalent 4.7 uF capacitor, a value too low to be considered useful as a bulk capacitor. This means that for larger capacitance, an in-rush current control is needed.
  3. Resettable fuses are fixed to a current limit and can’t be changed once in operation.
  4. Manual switches remove the option to implement software control of the power outputs.

Upstream Section

Fortunately, ICs exist today that help solve these issues in an efficient way, albeit with the penalty of a higher cost. For the upstream section, the section that implements the power combiner (D1 and D2), I chose TI TPS259470ARPW, an advanced eFuse, as the central piece to solve many problems at once:

Graph Two: Upstream power muxer with TPS259470s.

Downstream Power Control and Protection

The DIODES AP22653, a current-limited power switch, is used in each downstream port. This IC is specific for this application and provides the following features to USB Insight Hub:

Graph Three: Each port power control.

Power Path Losses

The overall power path design, though somewhat complicated, provides protection, control, and efficiency. At full load you can expect the voltage drop in any channel is at most 0.385 V, or an equivalent 192 mΩ. If only one channel is draining 2 A, the expected voltage drop is 0.238 V (120 mΩ).

CH1@2A, CH2@1A, CH3@2A. Total current = 5A. External power supply: Raspberry Pi 27W USB-C.

I hope this explanation provides a good understanding of the power path and the thorough process we followed to implement safe and high performance power delivery to the devices you work with.

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